The present invention relates to an optical information recording medium which is suitable for recording and reproducing information at high speed and at high density by changing some property of a recording film with use of light, heat and the like.
By focusing a laser beam through a lens system, a small light spot having a diameter on the order of the wavelength of the beam can be formed. Use of such light beam makes it possible to form a light spot the energy density per unit area of which is high, even with use of a light source of low output power. It is therefore possible to change the material in a very small area thereof and, in addition, to read the change in that very small area. An optical information recording medium was produced by making use of this technology. Hereinafter, the optical information recording medium will be referred to as "optical recording medium" or simply as "medium".
The optical recording medium has a basic structure comprising a substrate with flat surface and a recording film layer formed on the substrate so as to make some change in state with application of the laser beam spot. Recording and reproduction of a signal are carried out in the following manner. Namely, the laser beam is focused to be applied or irradiated onto the recording film surface of the medium of a plate shape which is moved by rotating means or translating means such as motor. The recording film absorbs the laser beam to result in the temperature rise. As the output power of the laser beam is made greater than a certain threshold value, the state of the recording film is changed to allow information to be recorded. This threshold value is a quantity which depends on the thermal characteristics of the substrate, relative transverse speed of the medium with respect to the light spot and the like, in addition to the characteristics of the recording layer itself. Reproduction of the recording information is carried out by applying onto the recorded portion the laser beam spot of an output power sufficiently lower than the above threshold value and, then, detecting difference between the recorded portion and the non-recorded portion in some optical characteristic such as the intensity of transmitted light, the intensity of reflected light or the direction of polarization of these lights.
For this reason, it is hoped to develop a structure and a material the state of which is changed with a small power of the laser so as to show a significant optical change.
There are known, as the recording film, metal films of Bi, Te, or containing Bi or Te as main ingredient, and compound film containing Te. These recording films are applicable to ablative recording in which the laser beam is applied to melt or evaporate a port of the film so as to form a small hole. Since the optical phase of the reflected light or transmitted light is difference between from the recorded portion and from its peripheral portion, the lights are cancelled each other due to destructive interference or diffracted so as to change the quantity of the reflected or transmitted light capable of reaching a detection system. Reproduction is effected by detecting this change. On the other hand, there is known another recording medium called structural phase change or phase transition type in which optical change takes place without causing any change in shape of the medium. There have been proposed as the recording film material an amorphous chalcogenide film and an oxide film containing Te-TeO.sub.2 as main ingredient (Japanese Patent Examined Publication No. 54-3725). There has also been known a thin film containing Te-TeO.sub.2 -Pd as main ingredient (Japanese Patent Unexamined Publication No. 61-68296). As the laser beam is applied to these films, at least one of the refractive index and the extinction coefficient of the film is changed so as to effect recording. Amplitude of the transmitted or reflected light is changed in this recorded portion. As a result, the quantity of the transmitted or reflected light capable of reaching the detection system is changed so that reproduction of signal is effected by detecting this change.
Light is wave and, accordingly, is characterized by amplitude and phase. As described above, reproduction of signal is detected in accordance with the change in the quantity of the transmitted or reflected light, this change being attributable to a change in the amplitude of the transmitted or reflected light in a very small area of the film itself (amplitude change or modulation recording) and to a change in the optical phase of the transmitted or reflected light (optical phase change or modulation recording). Incidentally, reflection coefficient means hereinafter a ratio of the light energy (that is, a square of amplitude) of the reflected light to that of the incident light.
In the ablative one of the optical recording mediums described above, the quantity of the reflected light is changed greatly and the optical phase modulation recording is carried out so that recording can be executed at high recording density. However, it is difficult to form regular holes and the noise level is high at the time of reproduction. Further, it is impossible to provide a contact Protective structure so that it is necessary to provide a complicated hollow structure called air-sandwiched structure, resulting in difficulty in manufacture and high cost. In addition, since deformation of recording layer is carried out upon recording, it is impossible to erase and rewrite.
To the contrary, the structural phase change type recording medium is not accompanied with deformation so that it can have a simple structure and can be manufactured easily and at low cost. However, there has conventionally been a problem that since amplitude modulation recording has been carried out the recording density is low as compared with ablative recording. Further, there is another problem that it is difficult to have compatibility with the write once type recording medium and the read only type replica disc (such as audio disc and video disc) which are optical phase change type recording media making use of concave/convex pits. In addition, there arises a further problem that when the of the geometrical deformation type recording region with the concave/convex pits and the structural phase change type recording region are made to coexist together in advance on one recording layer, reproducing signals from the both regions are differed from each other in the form having the recording in formation therein.